In some existing computer systems communication from a control unit to a peripheral device is normally accomplished serially using a bi phase encoded scheme. A protocol is selected to ensure that all bits have a transition at the mid-bit boundary. Heretofore, two methods have been implemented for data recovery, one called "data sampling" and the other termed "analog clock recovery".
Analog clock recovery energizes a tank circuit on all falling edges of the inbound transmission. The tank circuit is precision tuned to a frequency 8 times that of the transmitter, thus the transmission clock is recovered in "real time" and the inbound data can be synchronized to this clock. This system works satisfactorily except phase distortion can cause the recovered data to have skewed bit cells and the circuit presents an invariable impedance which itself contributes to distortion.
Digital sampling recovers data by measuring the value of the received data at regular intervals and then follows by decoding where each bit cell is located. The inherent problem of this system is that cells and bit time relationships must be determined in parallel.
This disclosure provides for digital data recovery that increases the quality and reliability of recovered data. These advantages are achieved since the received data is not sampled. Instead, data is decoded by measuring the time between consecutive falling edges of a data stream relative to a local time base. The time measurement is then used with a set of rules, or tables, to determine whether the falling edges occured on bit boundaries or mid-bit and further logic reveals the encoded data. The method of this disclosure allows liberal amounts of phase distortion because bit cell definition is, compared to the previously known systems, relatively unimportant.
The digital data recovery system of this disclosure has been developed to meet the need to inexpensively and reliably extract data from a Manchester encoded stream that typically exists between a cluster controller and peripherals, that is, terminals, printers, multiplexers, etc.
Manchester encoding is a protocol in which each bit, or bits cell, exhibits a self clocking property. The transmitter clock is related to the transition, that is, the rising or falling edge, in the center of each bit cell. At "1" is defined by a rising edge in the center of the bit cell and "0" is defined by a falling edge in the center of the bit cell.
The system of this disclosure is for deriving digital data from a bi phase encoded data stream wherein signal transition occurs at mid-bit or bit boundaries, the derived digital data being represented by 1's or 0's, includes an edge detector for detecting the occurrence of each failing edge of the data stream. An edge decoder is connected to an edge detector for determining the time between the successive failing edges, as well as qualifying falling edges.
A falling edge identifier is connected to the edge decoder for determining whether a detected falling edge occurs on a bit boundary (BB) or at mid-bit (MB).
A falling edge memory is connected to the falling edge identifier for remembering whether the previous falling edge occurred at BB or MD.
A data decoder is connected to receive input from the falling edge memory and the edge decoder. The data decoder has logic for determining the recovered data for passage out as decoded data, that is, as 1's and 0's, according to a table. There is further logic which implements a data strobe, indicating valid recovered data from the data decoder to any external circuits.
Thus, the system herein provides an improved system including improved algorithms for recovering digital bi phase data from a Manchester format data stream that is relatively inexpensive to produce in that it has relatively simple and inexpensive solid state components. Further, the system of this disclosure has increased quality and reliability and is relatively insensitive to phase distortion.
A better understanding of the invention will be had by reference to the following description and claims, taken in conjunction with the attached drawings.